A polymer electrolyte fuel cell (hereinafter referred to as a “PEFC”) causes an electrochemical reaction between a hydrogen-containing fuel gas and an oxygen-containing oxidizing gas, such as air, to generate electric power and heat at the same time. A unit cell (cell) of the PEFC includes a MEA (Membrane-Electrode Assembly), gaskets, and electrically-conductive plate-shaped separators. The MEA is constituted by a polymer electrolyte membrane and a pair of gas diffusion electrodes (an anode and a cathode). The PEFC is commonly formed such that a plurality of cells are stacked, these stacked cells are sandwiched between end plates, and the end plates and the cells are fastened by fastening members (see PTL 1, for example).
Therefore, the vicinity of a portion (hereinafter referred to as an “outer peripheral contact portion of the polymer electrolyte membrane”) of the polymer electrolyte membrane receives a stress (fastening pressure) higher than a stress applied to the other potion of the polymer electrolyte membrane, the portion contacting the outer periphery of a catalyst layer of the gas diffusion electrode. In addition, by repeatedly performing start-up and stop operations of the PEFC, a tensile stress and a compressive stress are repeatedly applied to the polymer electrolyte membrane. Especially, these stresses are strongly applied to the outer peripheral contact portion of the polymer electrolyte membrane, and the problem is that damages, such as distortions and cracks, tend to occur at this outer peripheral contact portion portion.